The response of the macroinvertebrate community to low-flow variability and supra-seasonal drought within a groundwater dominated stream

Information regarding the instream response to long supra-seasonal droughts, extending over more than one season, in largely perennial riverine systems is fragmentary compared to knowledge regarding floods. The macroinvertebrate community of a groundwater dominated river was investigated over an eight-year period which included two supra-seasonal drought events (1992 and 1996-1997). The community responded directly to changes in the discharge regime. Significant differences in the community abundance, the abundance of the amphipod Gammarus pulex, diversity indices (Shannon Wiener and Simpson diversity) and the Berger-Parker dominance index were recorded between drought, flow recovery and other (non-drought) years. Drought conditions resulted in extremely low community abundance, Gammarus pulex abundance and the Berger-Parker dominance index, and extremely high values of the Shannon Wiener and Simpson diversity indices. Recovery of the community extended over a two-year period and reflected recovery of flow associated with recharge of the groundwater aquifer. The results underline the continuing need for medium to long-term data series to investigate the response of instream communities to flow variability so that extreme events can be placed within the context of natural variability.